An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles dispersed in a dielectric solvent. An EPD typically comprises a pair of spaced-apart plate-like electrodes. At least one of the electrode plates, typically on the viewing side, is transparent. An electrophoretic fluid composed of a dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrode plates.
An electrophoretic fluid may have one type of charged pigment particles dispersed in a solvent or solvent mixture of a contrasting color. In this case, when a voltage difference is imposed between the two electrode plates, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color.
Alternatively, an electrophoretic fluid may have two (or more) sets of pigment particles of contrasting colors and carrying varying charges. Typically the sets of pigment particles are dispersed in a clear solvent or solvent mixture. In the case of t, when a voltage difference is imposed between the two electrode plates, the two types of pigment particles would move to opposite ends. Thus one of the colors of the two types of pigment particles would be seen at the viewing side.
An electrophoretic fluid may also comprise multiple types of charged pigment particles of different optical characteristics. The different types of charged pigment particles have different charge polarities and/or charge levels. Such a fluid may allow a display device to display multiple color states.
The charge behavior of each type of pigment particles depends on both the particle surface chemistry and the presence of charge control agent(s) in the fluid. The charge control agent in fact plays a critical role in controlling the optical performance of a display device.